1. Field of the Invention
The present invention relates to packaging that permits hermetic passage of one or more optical fiber(s) through a barrier wall. More particularly this invention relates to reliable and low cost hermetic seals for ribbonized (fiber counts greater than one) fiber optic cable feed-through.
2. Description of the Related Art
Packaging of fiber optic devices is driven by the need to achieve low cost while at the same time maintaining high reliability for extended operational life, for example in excess of 20 years. It is particularly important to achieve a hermetic seal around the fiber optic cable as it enters the device package. In this context, the hermetic seal is required to provide a good barrier to the ingress of moisture, which can cause damage to the fiber optic devices within the package.
According to one method for hermetically sealing of fiber optic cables, a glass to metal seal is formed between the fiber optic cable and its supporting tube or sleeve. This involves locally heating a stripped portion of the fiber optic cable to around 500 C to reflow a precision solder glass bead positioned at an appropriate place to form the seal. At the same time, the fiber jacket, or protective outer coating must be maintained to below 90 C to avoid damage to the coating. Control of the reflow profile is quite critical as high stress concentrations can lead to damage to the optical fiber and subsequent light attenuation in use.
According to another method, the fiber optic cable is coated with metal (for example titanium/platinum/gold or titanium/tungsten/gold) and then soldered to its support tube. This approach involves costly metal deposition processes to coat the individual fibers. It is also difficult to maintain adhesion of the coating to the fiber. In addition, as with the first method mentioned above, the required heating can lead to high variations of stress and deformation when processing multiple fibers.
In another type of fiber optic package, an outer sleeve of a material resistant to moisture ingress with an insert is located in the sleeve at one end having a through bore. An optical fiber extends through the bore and beyond the end portion of the sleeve with adhesive films respectively securing the insert to the sleeve and the optical fiber to the insert. This approach involves the use of epoxy as a moisture barrier. Epoxy is a viable short-term solution for creating a moisture barrier; however epoxy will eventually permit moisture diffusion through the package, making the bond line/moisture path an area of concern.
Thus, there is a need for improved hermetic seals for fiber optic packages, particularly seals having improved reliability and seals that are cost-effective and uncomplicated to assemble as fiber counts increase and package dimensions decrease.